1. Field of the Invention
The present invention relates to a capacitive sensor for measuring distance, in particular a capacitive sensor for measuring distance to a target in a lithography apparatus.
2. Description of the Related Art
Charged particle and optical lithography machines and inspection machines are used to expose patterns onto wafers and other targets, typically as part of a semiconductor device manufacturing process. In a lithography system a wafer is usually exposed at multiple locations by optical or particle exposure beams generated by the lithography machine. The wafer is usually positioned on a wafer table and multiple exposures are typically achieved by controlled displacement of the wafer table with respect to a stationary electron/optics column. The exposures are typically performed continuously on the wafer surface.
The wafer surface which is to be exposed is almost never completely flat. A typical wafer may have a bow in it of up to 50 μm without clamping to the wafer table. Apart from the wafer bow the wafer surface may have other non-uniformities over its surface. The wafer bow and other non-uniformities result in height variations in the wafer surface. To achieve the extremely high precision required of modern lithography machines, it necessary to correct for this height variation to maintain the wafer surface that is exposed in the focal plane of the projection lens used to focus the optical or particle exposure beams onto the wafer.
The wafer table that holds the wafer may be adjusted to compensate for these variations in height of the wafer surface. The height of the wafer table may be adjusted to bring the wafer surface to be exposed into the focal plane of the projection lens. Control of the wafer table height may be accomplished using signals transmitted from sensors which measure the height of the wafer surface, e.g. the distance between the projection lens and the wafer surface. Highly sensitive sensors are required to ensure correct control of wafer position at the extreme precision required for modern lithography machines. Various types of sensors have been used for this type of application, including capacitive probes. However, the existing capacitive probes and associated measurement and control systems have suffered from several drawbacks.
Existing capacitive sensors are typically large, both in height and sensor area. FIGS. 1A and 1B show the structure of a prior art capacitive sensor. FIG. 1A shows a cross-sectional view and FIG. 1B shows an end view of the sensor probe. A conductive sensing electrode 2 is surrounded by a conductive guard electrode 3. An insulating layer 4 separates the two electrodes and another insulating layer 5 may be used to separate the guard electrode 3 from the housing 6. An electrical cable 7 and connector 8 connects the sensor to a signal processing system to derive the desired final measurement signal. The operating range of the sensor is dependent upon the sensing area under the sensing electrode 2. The guard electrode 3 is set at the same potential as the sensing electrode to confine the electric field within the sensing area to generate a relatively uniform electric field between the sensing electrode 2 and the target 9. This type of construction leads to a relatively tall sensor, generally about 20 mm in height, and a relatively large sensing electrode.
The relatively large height and width of the sensors requires that the sensors need to be located relatively far from the projection lens, introducing errors due to variation in the relative positioning of the sensors and the projection lens due to manufacturing tolerances and thermal expansion. The relatively large size of existing capacitive probes also requires that individual sensors in multi-sensor configurations are spaced relatively far apart, reducing the spatial resolution of the sensing system so that non-uniformities in the wafer surface occurring over a small area of the wafer surface may not be detected. The relatively wide spacing also results in a slower measurement process, reducing throughput of a lithography machine using these systems.
British patent 2,131,176 describes a capacitance distance measuring probe manufactured by adhesively bonding together two thermoplastic polymer films with a copper coating deposited on one side, so that the copper coated face of one sheet is bonded to the uncoated face of the other sheet. The exposed copper coating on one sheet is divided into a first area which constitutes a sensing electrode and a second area which at least partially surrounds the sensing electrode and is electrically interconnected with a copper coating on the other sheet to define a guard electrode for the sensing electrode. This construction mimics the construction shown in FIG. 1 by providing a guard electrode surrounding the sensing electrode, both the guard electrode surrounding the sensing electrode formed on the same surface and at the same level of the layered device. This results in a structure which requires an electrical connection between different conductive layers and thus requires a more complex and costly manufacturing process.
Furthermore, the wiring connections to these sensors are difficult to make and the wiring introduces capacitances which affect the reading of the sensor and need to be taken into account, usually be calibrating the combined sensor and wiring installation. The requirement to calibrate existing sensors in combination with the sensor wiring requires recalibration whenever a sensor is replaced, making the replacement complex, time-consuming, and expensive.
U.S. Pat. No. 4,538,069 describes a method of calibrating a capacitance height gage for a single electron beam lithography machine for exposing reticles. The height gage is first calibrated in a calibration fixture using a laser interferometer, and the machine is then repositioned to the lithography station to expose a reticle and the distance to the reticle is measured with the capacitance gage. The capacitance gages are formed on a substrate which is secured to the bottom of the electron beam optics housing. The reticle target is grounded, the capacitance gages are driven by 180° out-of-phase signals, and the output signal from each gage is separately processed to generate four height measurement signals.